The present invention relates generally to methods and devices for drug delivery and analyte extraction, and specifically to medical methods and devices for puncturing the outer layer of living skin and to methods and devices for transdermal drug delivery and analyte extraction.
A number of different methods have been developed to perform transdermal drug delivery and/or analyte extraction, including passive diffusion of a drug or analyte between a skin patch and skin, as well as active processes such as iontophoresis, sonophoresis, electroporation, and chemically enhanced diffusion. These methods are primarily used for generating transdermal movement of small molecules, but generally do not enhance the motion of large molecules through the 10-50 micron thick outermost layer of the skin, the stratum corneum epidermidis.
In an article, xe2x80x9cMicromachined needles for the transdermal delivery of drugs,xe2x80x9d IEEE 11th Annual International Workshop on Micro-Electro-Mechanical Systems (1998), pp. 494-498, which is incorporated herein by reference, Henry et al. discuss a method of mechanically puncturing the skin with microneedles in order to increase the permeability of skin to a test drug. In the article, microfabrication techniques are described to etch an array of needles in silicon, and experiments performed on cadaver skin with the needle array demonstrated an increase in permeability subsequent to puncture of the skin. The needles are created with a predetermined length, and penetrate to the same depth from the skin surface, regardless of the local thickness of the stratum corneum. It is known that if the needles are longer than the local thickness, then the underlying epidermal tissue may be injured, while if the needles are too short, channel formation through the stratum corneum may be incomplete.
U.S. Pat. Nos. 4,775,361, 5,165,418, and 5,423,803, and PCT Publication WO 97/07734, the disclosures of which are incorporated herein by reference, describe methods of using laser pulses to locally heat the stratum corneum to about 120xc2x0 C., thereby causing local ablation, in order to cause a single hole to develop in the stratum corneum through which large molecules may pass. Whereas some selectivity of ablation depth can be attained by varying the wavelength of the laser pulse, no feedback mechanism is disclosed whereby the laser pulses are terminated upon generation of the necessary damage to the stratum corneum.
PCT Publication WO 97/07734 also discloses thermal ablation of the stratum corneum using an electrically resistive element in contact with the stratum corneum, such that a high current through the element causes a general heating of tissue in its vicinity, most particularly the stratum corneum. As above, no means are disclosed to terminate current flow upon sufficient disruption of the stratum corneum. Additionally, thermal characteristics of skin vary highly across different areas of an individual""s skin, as well as among a group of subjects, making optimal thermal dosages, which produce the desired ablation without causing pain, very difficult to determine. Lastly, increasing transdermal molecular flow by increasing the permeability of the stratum corneum, whether using microneedles, laser energy, or resistive heating of tissue, is inherently a two step process: (a) position apparatus to generate holes, and (b) apply a patch to the skin, through which the molecules will flow.
Electroporation is also well known in the art as a method to increase pore size by application of an electric field. This process is described in an article by Chizmadzhev et al., entitled xe2x80x9cElectrical properties of skin at moderate voltages,xe2x80x9d Biophysics Journal, February, 1998, 74(2), pp. 843-856, which is incorporated herein by reference. Electroporation is disclosed as a means for transiently decreasing the electrical resistance of the stratum corneum and increasing the transdermal flux of small molecules by applying an electric field to increase the size of existing pores. Electroporation generally does not produce pores of sufficient diameter to pass large molecules therethrough. Additionally, optimal voltage profiles are difficult to determine because of naturally occurring variations as described hereinabove, as well as the lack of an accurate feedback mechanism to indicate achievement of the desired pore enlargement. If excessive voltage is applied, an irreversible breakdown occurs, resulting in damage to the skin and possible sensations of pain.
U.S. Pat. No. 5,019,034 to Weaver et al., whose disclosure is incorporated herein by reference, describes apparatus for applying high voltage, short duration electrical pulses on the skin to produce electroporation, and states that xe2x80x9c. . . reversible electrical breakdown . . . along with an enhanced tissue permeability, is the characteristic effect of electroporation.xe2x80x9d
It is an object of some aspects of the present invention to provide improved apparatus and methods for transdermal delivery of an active substance.
It is a further object of some aspects of the present invention to provide improved apparatus and methods for transdermal analyte extraction.
It is yet a further object of some aspects of the present invention to provide improved apparatus and methods for creating narrow channels through the stratum corneum of living skin by puncturing.
It is still a further object of some aspects of the present invention to provide improved apparatus and methods for reducing sensation and minimizing damage to skin underlying the stratum corneum during channel creation.
It is an additional object of some aspects of the present invention to provide improved apparatus and methods for controlling the timing of channel creation.
It is yet an additional object of some aspects of the present invention to provide improved apparatus and methods for regulating channel creation responsive to properties of the skin.
It is another object of some aspects of the present invention to provide improved apparatus and methods for puncturing the skin and/or transdermally delivering an active substance and/or transdermally extracting an analyte, using a miniature, self-contained device.
It is yet another object of some aspects of the present invention to provide improved apparatus and methods for transdermally delivering an active substance using a standard medical skin patch.
In preferred embodiments of the present invention, a device for enhancing transdermal movement of a substance comprises: (a) a skin patch, with at least two electrodes in contact with the skin of a subject; and (b) a control unit, coupled to the patch, which causes a current to pass between the electrodes through the stratum corneum epidermidis, in order to generate at least one micro-channel in the stratum corneum to enable or augment transdermal movement of the substance. Preferably, the control unit comprises switching circuitry to control the magnitude and/or duration of the electric field at the electrode.
The term xe2x80x9cmicro-channelxe2x80x9d as used in the context of the present patent application and in the claims refers to a pathway generally extending from the surface of the skin through all or a significant part of the stratum corneum, through which pathway molecules can diffuse. Preferably, micro-channels allow the diffusion therethrough of large molecules at a greater rate than the same molecules would diffuse through pores generated by electroporation. It is believed that such micro-channels are formed due to local power dissipation leading to ablation of the stratum corneum when an electric field of sufficient magnitude is applied to a small area of the skin, in contact with the electrodes, for a certain period of time. Unlike methods of electrically-promoted drug delivery known in the art, such as iontophoresis and electroporation, the present invention enables relatively large channels to be formed, through which even large molecules of the active substance can pass rapidly, without the necessity of ionizing or polarizing the molecules.
The current flow between the electrodes can be described as having two components: (a) a perpendicular component, which is generally perpendicular to the skin surface (and, if the associated electric field is sufficiently large, may cause current to go through the stratum corneum into the underlying epidermal tissue and dermis); and (b) a lateral component, generally parallel to the skin surface, which remains generally within the stratum corneum. Substantially all of the current generated at one electrode ultimately emerges from the skin and is taken up by an adjacent electrode.
In preferred embodiments of the present invention, methods and/or apparatus are employed to increase the relative value of the lateral component with respect to the perpendicular component. In general, the stratum corneum epidermidis (the superficial layer of the epidermis) demonstrates a significantly higher resistance to the passage of molecules therethrough than does the underlying epidermal tissue. It is therefore an object of these preferred embodiments of the present invention to form micro-channels in the stratum corneum by ablating the stratum corneum in order to increase conductance of the substance therethrough, and to generally not directly affect or damage epidermal tissue underlying the stratum corneum or in the innervated dermis. Additionally, limiting current flow substantially to the non-innervated stratum corneum is expected to decrease or eliminate the. subject""s sensations, discomfort, or pain responsive to use of the present invention, particularly as compared with other procedures known in the art.
A voltage applied between two electrodes on the skin generates an electric field that is to a large extent confined to the volume in a vicinity of the electrodes. Thus, electrodes which are widely spaced produce a field and current flow responsive theretoxe2x80x94which extends relatively deep into the skin. Conversely, electrodes which are closely spaced do not generate significant current flow at deeper layers. Therefore, in some preferred embodiments of the present invention, the electrodes of the device are separated by distances smaller than about 100 microns (but for some applications by distances of up to approximately 500 microns), in order to generate a current flow which is largely confined to a thin layer, comprising most or all of the stratum corneum. This effectively results in a desired larger value of the ratio of the lateral component to the perpendicular component, as described hereinabove.
In some of these preferred embodiments of the present invention, a high-frequency AC current with an optional DC current added thereto is applied between the closely-spaced electrodes in order to generate lateral capacitive currents in the stratum corneum and to cause breakdown and micro-channel formation in the stratum corneum.
In some preferred embodiments of the present invention, the patch comprises an array of electrodes, preferably closely-spaced electrodes, which act together to produce a high micro-channel density in an area of the skin under the patch. Preferably, the control unit and/or associated circuitry sequentially or simultaneously evaluates the current flow through each electrode, or a subset of the electrodes, in order to determine when one or more micro-channels have formed responsive to the applied field. Responsive thereto, the control unit discontinues application of the field. Since the formation of a micro-channel is typically marked by a local drop in electrical resistance of the skin, the control unit may, for example, reduce the voltage or current applied at any electrode wherein the current has exceeded a threshold. By reducing current flow upon or shortly after micro-channel formation, the likelihood of skin burns or pain sensations is minimized.
In some preferred embodiments of the present invention, a relatively high voltage is applied to the electrodes initially, so as to induce formation of micro-channels through the skin. A property of the current flow is detected, and the current is reduced or terminated when the property reaches a predetermined threshold. Preferably, the detected property of the current flow is secondary to changes in a conduction property of the skin, responsive to formation of one or more micro-channels through the stratum corneum.
Alternatively or additionally, a time-varying voltage V(t), characterized, for example, by the formula V(t)=V0+ktn, is applied between a first electrode and a second electrode in the skin patch until a shut-off signal is generated. (Constants k and n are nonnegative.) Other forms of V(t) may include a sinusoid, an exponential term, or a series of pulses. A current I(t), flowing responsive to the applied field, is measured by the control unit, as described hereinabove. Calculations of the values of ∫I(t)dt, dI/dt and/or d2I/dt2 are frequently performed. Comparisons of I and/or ∫I(t)dt and/or dI/dt and/or d2I/dt2 with respective threshold values are used as indicators of micro-channel formation and/or to determine when to generate the shutoff signal for the electrodes.
Further alternatively or additionally, in embodiments in which V(t) is sinusoidal, the control unit preferably calculates changes in a phase shift between V(t) and I(t) during application of the electric field, and controls the field responsive to these changes. It is believed that cells in the stratum corneum demonstrate capacitance, which causes the phase shift, and that ablation of the stratum corneum decreases the capacitance and is evidenced by a decrease in the phase shift.
Still further alternatively or additionally, the total charge which is passed through the skin is limited by a capacitor, inductor, or other energy-storage device. An appropriate choice of values for these components sets an absolute maximum quantity of charge which can pass through the skin, and thus limits any damage that can be caused thereby.
In some preferred embodiments of the present invention, one or more of the electrodes comprise or are coupled to an electrically conductive dissolving element, where the dissolving rate is generally proportional to the current passing through the electrode. When a sufficient quantity of charge has passed through the dissolving element, the electrode ceases to conduct electricity. Thus, a maximum total charge, Qtotal, is associated with an electrode, such that current flows through the element for only as long as q(t)=∫I(t)dt less than Qtotal. This serves as a safety feature, reducing the possibility of skin burns secondary to applied electric fields. Alternatively or additionally, the dissolving element is constructed so that it becomes non-conductive after a quantity of charge has passed therethrough which is sufficient to ablate the stratum corneum.
In some further preferred embodiments of the present invention, the electrodes are xe2x80x9cprintedxe2x80x9d directly on the skin, preferably by stamping or by employing a transfer patch of a conductive substance (such as, for example, a conductive ink containing silver grains). In applications of such embodiments of the present invention for transdermal drug delivery, the conductive substance preferably comprises a matrix holding the drug to be administered to a subject.
Preferably, the printed electrodes demonstrate a substantially complete loss of conductance therethrough upon ablation of the stratum corneum responsive to the applied electric field. Further preferably, each printed electrode comprises a material which is conductive only when current flowing therethrough remains below a threshold value. If the current exceeds the threshold, then thermal fusion of the material causes it to become largely nonconductive, i.e. the material acts as a fuse. Still further preferably, current continues to flow through the other electrodes until they reach the threshold current, at a time which is generally associated with the time required for ablation of the stratum corneum, as described hereinabove. In some of these embodiments, the control unit may be made substantially simpler than as described regarding other embodiments, and generally does not need other circuitry in order to determine whether to generate a shut-off signal.
In still further preferred embodiments of the present invention, two electrodes on the patch form a concentric electrode pair, in which an inner electrode generates a current which passes through the stratum corneum to an outer electrode surrounding the inner electrode. The distance between the inner and outer electrodes is preferably between about 50 and about 200 microns, in order to maintain the ratio of the lateral to the perpendicular component of the current at a high value, as described hereinabove.
In some preferred embodiments of the present invention, a conductance-enhancing substance, preferably comprising a conductive cream or ink, is applied to the skin in order to increase the ratio of the lateral to the perpendicular component of current flow. Alternatively or additionally, the conductance-enhancing substance comprises a composition with a high diffusion coefficient, which diffuses into the lipid layers of the stratum corneum and further augments the selective power dissipation therein, in order to ablate the stratum corneum with substantially little damage to the underlying tissue. In some applications, the substance has an electrical charge associated therewith, such that when a small lateral field is applied, lateral diffusion of the substance within the stratum corneum is enhanced (i.e., iontophoresis of the substance).
In some of these preferred embodiments which utilize a conductance-enhancing substance, the substance further comprises an active substance, for example, a pharmaceutical product, dissolved or mixed therein. Since breakdown of the stratum corneum is often associated with removal of the enhanced conductivity path afforded by the conductance-enhancing substance, it is preferable in many of these embodiments to use a substantially constant voltage source to generate current at the electrodes. Removal of the enhanced conductivity path will result in a desired reduced power dissipation in the stratum corneum (P=V2/R), since the voltage remains constant while resistance increases.
In other preferred embodiments of the present invention, ablation of the stratum corneum is accomplished using a current-limited source to power the electrodes. It is believed that the stratum corneum generally displays high electrical resistance, while epidermal tissue underlying the stratum corneum has significantly lower electrical resistance. Ablation of the stratum corneum (i.e., removal of the high-resistance tissue) is therefore associated with a net decrease of electrical resistance between the electrodes, and the power dissipated in the epidermis following electrical breakdown will decrease, typically proportional to the change in resistance (P=I2R).
Monitoring changes in voltage, current, and/or phase for each electrode in the control unit may require, in certain implementations, a significant amount of circuitry. Therefore, in some preferred embodiments of the present invention, the control unit comprises one or more clusters of electrodes, in which monitoring and control are performed for each cluster rather than for the individual electrodes therein. The cluster is preferably over a relatively small area of skin, for example, from about 1 mm2 to about 100 mm2, in which properties of the skin are assumed to be substantially constant.
In some preferred embodiments of the present invention, the device is a stand-alone device, which enables transdermal delivery of an active substance or enhances transdermal motion of an analyte. Alternatively, the device creates micro-channels as described hereinabove and is then removed from the skin, in order to enhance the transdermal delivery of a substance into or out of a commercially-available skin patch subsequently placed on the skin. In other preferred embodiments of the present invention, the device is an add-on to commercially available transdermal drug delivery/analyte extraction devices, and serves primarily to create the micro-channels in the stratum corneum, and optionally to act as a vehicle through which the substance may pass.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a device for ablating the stratum corneum epidermidis of a subject, including:
a plurality of electrodes, which are applied to the subject""s skin at respective points; and
a power source, which applies electrical energy between two or more of the plurality of electrodes, to cause ablation of the stratum corneum primarily in an area intermediate the respective points.
Preferably, skin layers beneath the stratum corneum are substantially not ablated. In a preferred embodiment of the present invention, the ablation of the stratum corneum causes a puncturing thereof.
Preferably, the device ablates the area of the stratum corneum in order to allow a substance to pass therethrough. Further preferably, the substance includes a drug that is delivered through the skin. Alternatively, the substance includes an analyte that is extracted through the skin.
Preferably, the power source generates an electric field which causes a current to flow through the stratum corneum, and the device reduces power dissipated in the stratum corneum responsive to variation of a characteristic of the current. Further preferably, the characteristic is drawn from a list consisting of: the magnitude of the current; a time-integration of the current; a first time-derivative of the current; and a second time-derivative of the current.
Alternatively or additionally, current through one of the plurality of electrodes is reduced substantially independently of the current through another one of the plurality of electrodes.
Further alternatively or additionally, at least one of the plurality of electrodes is coupled to receive the current through a limited-conduction unit, which unit allows current below a threshold value to flow therethrough substantially unimpeded, and becomes substantially non-conductive if the current flowing therethrough exceeds a limited-conduction threshold value.
Preferably, at least one of the plurality of electrodes is coupled to an electrically-conductive dissolving element characterized by a dissolving rate generally proportional to the current passing therethrough, which becomes substantially nonconductive responsive to a function of the current. In a preferred embodiment, the function includes a time-integral of the current having passed through the dissolving element.
Preferably, the dissolving element includes:
an electrolyte solution within the element; and
a first node and a common node immersed in the electrolyte solution,
wherein current flows from the first node to the common node through the electrolyte solution, the current flow causing the common node to be consumed at a rate generally proportional to the current passing therethrough; such that the dissolving element becomes substantially nonconductive when the total charge having passed through the common node exceeds a common node threshold value.
Further preferably, the dissolving element also includes a second node, immersed in the electrolyte solution, wherein the power source generates alternating current, and wherein the device further includes:
a first diode, coupled in series between the power source and the first node, which conducts current from the power source to the first node when the alternating current is in a positive phase thereof; and
a second diode, coupled in series between the power source and the second node, which conducts current from the second node to the power source when the alternating current is in a negative phase thereof,
such that the dissolving element becomes substantially nonconductive when the total charge having passed through the common node exceeds a common node threshold value.
Alternatively, the dissolving element includes:
an electrolyte solution within the element;
a large-area anode immersed in the electrolyte solution; and
a plurality of cathodes, immersed in the electrolyte solution, each of the cathodes being coupled to a respective one of the plurality of electrodes,
wherein current flows from the large-area anode to the plurality of cathodes through the electrolyte solution, the current flow causing at least one of the cathodes to be consumed at a rate generally proportional to the current passing therethrough, and wherein the at least one cathode becomes substantially nonconductive responsive to a function of the current having passed therethrough.
Preferably, the current through at least one of the plurality of electrodes is reduced responsive to the variation of the characteristic of the current through another one of the plurality of electrodes.
In a preferred embodiment, the device also includes a voltage sensing unit coupled to measure a voltage drop across two of the plurality of electrodes, and current from the power source is reduced responsive to the measurement made by the sensing unit. Preferably, the power source includes a current source, and current from the current source is reduced responsive to a measurement made by the sensing unit which indicates that the electrical potential between the two electrodes is below a voltage-threshold value.
Alternatively or additionally, the device includes a resistive element coupled to one of the two electrodes and to the power source, and the voltage sensing unit is further coupled to measure a voltage drop across the resistive element in order to determine a current passing therethrough. The power source includes an alternating current source, such that the measurements of the voltage drop across the two electrodes and the current through the resistive element determine a phase shift. The current from the alternating current source is reduced responsive to the phase shift being below a threshold value.
In a preferred embodiment, the device includes:
a resistive element coupled to one of the plurality of electrodes and to the power source; and
a voltage sensing unit coupled to measure a voltage drop across the resistive element in order to determine a current passing therethrough.
In this preferred embodiment, the power source includes a voltage source, and the voltage is reduced responsive to a measurement made by the sensing unit which indicates that the current through the resistive element is above a current-threshold value.
In another preferred embodiment, at least one of the plurality of electrodes is printed directly on the skin and becomes substantially electrically nonconductive responsive to the value of the current passing therethrough being greater than a threshold value.
In yet another preferred embodiment, the device includes:
a capacitor, coupled to two of the plurality of electrodes; and
a switch, coupled to the power source and the capacitor, such that the switch, in a closed phase thereof, allows current to flow from the power source to the capacitor and to the two electrodes, and such that the switch, in an open phase thereof, substantially terminates current flow from the power source to the capacitor and to the two electrodes.
In this embodiment, the power source charges the capacitor during the closed phase, and the capacitor discharges current through the electrodes during the open phase.
Preferably, the distance between two of the plurality of electrodes is less than about 0.3 mm. Further preferably, the distance between two of the plurality of electrodes is between about 0.01 mm and about 0.1 mm.
Preferably, the plurality of electrodes include:
a common electrode, which has a plurality of perforations therethrough; and
a plurality of positive electrodes, each positive electrode passing through a respective perforation of the common electrode,
such that current from the power source flows from each positive electrode through the skin to the common electrode.
In a preferred embodiment, the power source generates alternating current, a frequency thereof being above about 100 Hz. Preferably, the frequency is between about 1 kHz and about 300 kHz. Alternatively or additionally, the power source modulates a frequency of the alternating current between a first frequency value and a second frequency value.
There is further provided, in accordance with a preferred embodiment of the present invention, a device for passing electrical current through the skin of a subject, including:
a power source, which generates the current;
a plurality of electrodes, which are applied to the skin at respective points; and
an electrically conductive dissolving element coupled to at least one of the electrodes, the element being characterized by a dissolving rate generally proportional to the current passing therethrough, and becoming substantially nonconductive responsive to a function of the current passing therethrough.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for ablating the stratum corneum epidermidis of a subject, including:
placing a plurality of electrodes against the subject""s skin at respective points; and
applying electrical energy between two or more of the plurality of electrodes, in order to cause ablation of the stratum corneum primarily in an area intermediate the respective points.
Preferably, skin layers beneath the stratum corneum are substantially not ablated. In a preferred embodiment, applying the electrical energy includes puncturing the skin.
Preferably, applying the energy includes ablating the area of the stratum corneum in order to allow a substance to pass through the area. Further preferably, the method includes delivering a drug or extracting an analyte through the area.
Preferably, applying electrical energy includes:
causing a current to flow through the points on the skin; and
substantially reducing the current flow through the skin responsive to variation of a characteristic of the current.
In a preferred embodiment, the characteristic is drawn from a list consisting of: a magnitude of the current; a time-integration of the current; a first time-derivative of the current; and a second time-derivative of the current.
In another preferred embodiment, causing the current to flow includes passing current to the one or more points on the skin through one or more respective limited-conduction units, wherein the units allow current below a threshold value to flow therethrough substantially unimpeded, and wherein the units become substantially nonconductive if the current flowing therethrough exceeds a limited-conduction threshold value.
In another preferred embodiment, causing the current to flow includes passing current to the one or more points on the skin through one or more respective electrically conductive dissolving elements, each element characterized by a dissolving rate generally proportional to the current passing therethrough, and each element becoming substantially nonconductive when the total charge having passed therethrough exceeds a dissolving element threshold value.
In yet another preferred embodiment, reducing the current flow includes reducing the current at one of the respective points substantially independently of the current at another one of the respective points.
In still another preferred embodiment, reducing current flow includes:
monitoring the current flow through one of the plurality of electrodes; and
reducing the current flow through another one of the plurality of electrodes responsive thereto.
Preferably, placing the plurality of electrodes includes placing two of the plurality of electrodes at a separation therebetween that is less than about 0.3 mm. Further preferably, placing the plurality of electrodes includes placing the two of the plurality of electrodes at a separation between about 0.01 mm and about 0.1 mm.
In a preferred embodiment, placing the plurality of electrodes includes:
applying a conduction-enhancing material to an area on the surface of the subject""s skin in order to enhance current flow through the skin; and
placing the electrodes on the material,
wherein the electrical resistance of the conduction-enhancing material increases responsive to a function of the current flow therethrough.
Preferably, placing the plurality of electrodes includes:
placing on the skin a common electrode which has a plurality of perforations therethrough; and
placing on the skin a plurality of positive electrodes, each positive electrode passing through a respective perforation in the common electrode, such that current from the power source flows from each positive electrode through the skin to the common electrode.
Further preferably, the method includes positioning in a vicinity of the electrodes a medical patch containing the substance, such that ablation of the stratum corneum increases a transport rate of the substance from the patch into the skin.
In a preferred embodiment, applying electrical energy includes generating alternating current, a frequency thereof being above about 100 Hz. Preferably, the frequency is between about 1 kHz and about 300 kHz. Alternatively or additionally, applying electrical energy includes modulating a frequency of the alternating current between a first frequency value and a second frequency value.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for passing electrical current through the skin of a subject, including:
placing a plurality of electrodes against the skin at respective points; applying the current through the electrodes; and
coupling to at least one of the electrodes an electrically conductive dissolving element, the element being characterized by having a dissolving rate generally proportional to the current passing therethrough, and by becoming substantially nonconductive responsive to a function of the current passing therethrough.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which:
FIG. 1A is a schematic, partly sectional illustration of a device for transdermal transport of a substance, in accordance with a preferred embodiment of the present invention;
FIG. 1B is a schematic, partly sectional illustration of another device for transdermal transport of a substance, in accordance with a preferred embodiment of the present invention;
FIG. 2 is a schematic bottom view of the device of FIG. 1A, in accordance with a preferred embodiment of the present invention;
FIG. 3 is a schematic illustration of a switching unit in the device of FIG. 1A, in accordance with a preferred embodiment of the present invention;
FIG. 4 is a schematic illustration of an electrode assembly, in accordance with a preferred embodiment of the present invention;
FIG. 5 is a schematic illustration of another electrode assembly, in accordance with a preferred embodiment of the present invention;
FIG. 6 is a schematic illustration of yet another electrode assembly, in accordance with a preferred embodiment of the present invention;
FIG. 7 is a schematic illustration of still another electrode assembly, in accordance with a preferred embodiment of the present invention;
FIGS. 8A and 8B are schematic illustrations of charge-limited electrode assemblies, in accordance with preferred embodiments of the present invention;
FIG. 9 is a schematic illustration of another charge-limited electrode assembly, in accordance with a preferred embodiment of the present invention;
FIG. 10 is a schematic illustration of yet another charge-limited electrode assembly, in accordance with a preferred embodiment of the present invention;
FIG. 11A is a schematic side view of a concentric electrode assembly, in accordance with a preferred embodiment of the present invention; and
FIG. 11B is a schematic top view of a common electrode layer in the concentric electrode assembly of FIG. 11A, in accordance with a preferred embodiment of the present invention.